(The Hill) — Departures of domestic flights are resuming across the country after the Federal Aviation Administration (FAA) restored a system that warns pilots of hazards during their upcoming flights.

The FAA first issued an advisory just past 4 a.m. Wednesday that a technical issue impacted its Notice to Air Missions system (NOTAM), and it announced a couple of hours later that it had ordered all airlines to pause domestic departures until 9 a.m. to ensure flight safety.

Thousands of flights were delayed, and hundreds have been canceled, but airlines are gradually restoring normal operations.

Here are five things to know about the system outage at the FAA:

The FAA is assessing the cause of the initial problem

A display shows flights, many delayed, at LaGuardia Airport in New York. A computer outage at the FAA brought flights to a standstill across the U.S. (AP Photo/Seth Wenig)

The FAA said it continues to look into the cause of the initial problem. President Joe Biden on Wednesday reiterated that “they don’t know what the cause is.”

At 6:30 a.m. Wednesday, the FAA said it was reloading the NOTAM system, which warns pilots of potential hazards such as closed runways and birds. The outage affected operations across the National Airspace System. 

The FAA grounded all planes while it was working to restore the system and repopulate it. Over a few hours on Wednesday, airlines were ordered to pause all domestic departures so the agency could “validate the integrity of flight and safety information.”

The administration has not seen evidence of a cyberattack

Transportation Secretary Pete Buttigieg said that the Biden administration has not yet ruled out the presence of nefarious activity in the FAA’s computer outage. (AP Photo/Susan Walsh, File)

The White House said while flights were still grounded that it had not seen evidence of a cyberattack being the cause of the system outage. Biden spoke to Transportation Secretary Pete Buttigieg, telling him to report directly back when the department learned the cause.

Biden said just before 8 a.m. that the FAA would have a “good sense” of what caused the issue within a couple of hours.

“There is no direct indication of any kind of external or nefarious activity, but we are not yet prepared to rule that out,” Buttigieg told MSNBC on Wednesday.

White House press secretary Karine Jean-Pierre also wrote on Twitter that “there is no evidence of a cyberattack at this point, but the President directed DOT to conduct a full investigation into the causes.”

Buttigieg said he directed the FAA to conduct an “after-action process” to determine the causes of the glitch and make recommendations for next steps to be taken. 

Flights are back up and running

Flights officially resumed just before 9 a.m. on Wednesday, though the ground stop has continued to affect certain departures throughout the day. (Getty Images)

The ground stop was lifted just before 9 a.m. Wednesday, and all domestic flights gradually resumed operating. 

Departures resumed at two particularly busy airports, Newark Liberty and Atlanta Hartsfield-Jackson, about 30 minutes earlier to clear up air traffic congestion in those areas.

But the ground stop seemed to have had an impact on domestic flights for at least the rest of the morning, if not longer. More than 7,000 flights within, into or out of the United States were delayed Wednesday, and more than 1,000 were canceled, according to the flight-tracking website FlightAware.

Reuters reported that more than 21,000 flights were scheduled to depart U.S. airports on Wednesday.

Travelers are likely to face delays throughout the day or longer

The unexpected outage may continue to affect travelers in the coming days. (Kevin Dietsch/Getty Images)

With hundreds of flights across the U.S. delayed as a result of the system outage, it will likely take a day or more for flight operations to resume as usual.

The flight disruption on Wednesday follows massive Southwest Airlines cancellations two weeks prior when the airline canceled more than half of its flights over a three-day period after Christmas and thousands of passengers were left stranded.

The issue occurred after the airline’s scheduling system struggled to route pilots and flight attendants to the right planes following strong winter storms that hit Denver and Chicago. Southwest CEO Bob Jordan said the company’s scheduling system needs to be modernized.

The outage prevented flights from taking off, not landing

Given that planes were permitted to land during the outage, there was no real danger for travelers already in the air as the technical issues were occurring. (AP Photo)

There was no real danger to flying during the system outage because flights were grounded. 

Planes that were already in the air were safe to land, but those still on the ground could not take off safely.

The FAA tweeted that pilots review the NOTAM system before they take off, so they would be aware of any possible hazards in advance of departure.

Multiple airports internationally also said that they were able to continue flights into the U.S. without any interruption. NBC reported that spokespeople for Air France, Frankfurt Airport in Germany and Gatwick Airport in London said they were still operating to and from the U.S.